The present invention relates generally to erosion control devices and methods adapted to prevent or reduce erosion of shoreline and near shore beach sediment or to allow beach sediment to accrete along the shoreline and in the near shore regions. More particularly, this invention pertains to a computer modeling methods used in the design and installation of geotextile groin fields in near shore regions to reduce the erosive effects of sediment transport of alongshore and on shore currents and to promote accretion of sediments in the near shore regions.
The world's system of beaches is a thin band of sand that is an environmental treasure. This system requires protective measures to sustain this valuable resource. The effects of reduced beach sediment input into the beach system will cause erosion and will have a negative impact on the natural process of beach morphology, thus reducing habitat space and protection for upland regions. The United States has approximately 17,000 miles of shoreline. Of this, approximately 10,000 miles are recreational beaches. The sand beaches of the United States are narrow and fragile and any loss or reduction in their characteristics would be catastrophic, both to nature and to the economy.
Most beach erosion can be attributed to the reduction of beach sediment input into the near shore systems due to anthropogenic effects. Sea ports, urbanized estuaries, river navigation and flood control projects, and even projects intended to prevent beach erosion can and do cause erosion. In one typical situation, a pier or jetty is constructed and extends perpendicular from the shoreline into the water. Littoral or near shore currents impinge upon the sides of the pier deflecting the currents away from shore. These currents typically carry sand which would otherwise be deposited near shore between naturally occurring sand bars extending parallel to the shore and the beach. However, since the currents are deflected away from shore, the sand is carried out to deep water, robbing the beach area of sand which would otherwise deposit there.
Furthermore, the deflected currents may wash away protective sandbars. Sandbars are critical to beach protection since they dissipate waves and littoral currents. When sandbars erode, the beachfront and the area of the eroded sandbar is exposed to much stronger currents and waves, causing even more severe beach erosion. Beachfront property owners often spend tens of thousands of dollars each to construct seawalls or revetments on and parallel to the beach in an attempt to stop such erosion. Such attempts, however, serve only to accelerate erosion. Seawalls and revetments only direct the energy of the waves and currents downwardly to the foundation of the seawall or revetment, which scours sand and rock at the foot of the seawall or revetment structure and which ultimately causes the structure to fall into the water. Such downwardly scouring also deepens the water in the area and allows sediment to be carried away from the littoral zone, leading to even more severe erosion.
While the negative effects of the foregoing can be mitigated to some degree, they can't be stopped. However, beach erosion can be retarded and managed. There are currently two basic choices with respect to beach erosion: (1) Allow the beach to erode; or, (2) nourish the beach with sand from a remote source. The former is generally unacceptable and the latter is expensive, but generally necessary. It is estimated that the federal government spends approximately $150,000,000 per year on shore protection and even more on collateral projects that affect beach systems. Individual states and localities spend more that twice that amount. The vast majority of these funds are invested in beach nourishment/re-nourishment projects. Beach nourishment can cost from $1 million to $5 million per mile of beach. Recurring maintenance costs (including re-nourishment) vary from $200,000 to over $1 million per year per mile depending on the re-nourishment cycle, which can vary from 3 to 5 years on average. If the re-nourishment cycle can be extended to 5 to 10 years, significant savings can be obtained.
Near-shore oceanographic studies have identified the complicated and diverse influences of coastal structures on inshore circulation. These influences may alter the character of the flow field including wave action, current speed and direction, turbulent energy levels and mean water levels. Clearly, wave characteristics are the predominant forcing influence on beach characteristics, such as beach profile and sediment grain size. However, man-made structures may influence beach characteristics as much as the natural forcing factors that shape the beach.
Groins fields are sometimes constructed on the beach and near shore region to trap and retain sand, and to nourish the beach compartments between them. Groins initially interrupt the alongshore transport of littoral drift. In conventional applications, they are most effective where alongshore transport is predominantly in one direction, and where their action will not cause unacceptable erosion of the downdrift shore. When a well placed groin field fills to capacity with sand, alongshore transport continues at about the same rate as before the groins were built, and a stable beach is maintained.
One known method of groin field construction involves placing anchored geotextile tubes on anchored mats. The geotubes are positioned one the shore and extend perpendicular to the shoreline into the water. The geotubes are then filled with a cement ballast material pumped into the tube from the shore. Where the currents exceed the erosion velocity, the weighted geotubes are positioned sufficiently far below the surface of the water such that the currents are forced to move upwardly over the geotubes, thereby reducing the velocity of the currents below the erosion velocity. The geotubes are positioned such that the waves associated with the currents do not reflect downwardly toward the bottom to scour the bottom. As the currents and waves rise over the geotubes, they will dissipate and slow down. They do not cause sand or other material to be carried to deeper water or undermine the erosion control structure. Because the currents can be slowed by the structures, sand will actually deposit between a plurality of such geotube structures positioned parallel to one another, ultimately burying the structures and increasing the beach area.
Currently, design and placement of geotube groin fields are based on rules of thumb and trial and error. Only rudimentary measurements of beach and near shore parameters are considered in designing a groin field. Specifically, design and adaptation of conventional geotube groin fields incorporate little in the way of predictive analysis of beach parameters and optimization of groin parameters.
What is needed, then, are numerical model simulations to replicate the beach system and to estimate the effect of varied groin field designs for erosion control.
What is also needed, then, is a method of evaluating multiple model executions of numerical model simulations to produce an engineering design of a discrete, porous, geotextile groin field for engineering site adaptation and implementation at a specific beach site.